1. Field of the Invention
The present invention relates to an optical switch having an optical waveguide whose output path of an optical signal branches into two, in which an output path for outputting the optical signal is switched according to a refractive index change caused by injecting carrier at a branching portion of the optical waveguide. More particularly, the invention relates to a semiconductor optical switch of the optical waveguide type enabled to highly increase an optical response speed.
2. Description of the Related Art
Current communication networks, such as LAN (Local Area Network) and WAN (Wide Area Network), usually employ communication systems, which transmit information through electrical signals.
Communication methods, which transmit information through optical signals, are employed only in trunk networks, which transmit large quantities of data, and in some other networks. Incidentally, these networks use “point-to-point” communication. Under the current situation, these networks have not developed to the level of a communication network, which is what is called a “photonic network”.
Realizing such a “photonic network” requires devices, such as an “optical router” and an “optical switching hub”, which have functions similar to those of a router and a switching hub that are used for switching the destinations of electrical signals. Also, measuring apparatuses for optical communication, which perform measurements of these devices, are required.
Further, such apparatuses (an optical system and the measuring apparatuses for optical communication) require optical switches each for switching a transmission path at a high speed. Hereinafter, a description is given of a conventional optical switch for switching the transmission path of an optical signal by forming an optical waveguide in a semiconductor and by providing a current injection region therein and by injecting an electric current (carriers) into the semiconductor to thereby change the refractive index of the region.
FIGS. 3 and 4 are a plan view and a cross-sectional view, respectively, showing an example of the conventional optical switch (See, for example, Document (1) referred to below.). As shown in FIG. 3, an “X-shaped” optical waveguide 2 is formed in a substrate 1. An electrode 3 is formed at an intersecting portion of the “X-shaped” optical waveguide 2. An electrode 4 is formed in the vicinity of the intersecting portion of the “X-shaped” optical waveguide 2 in parallel with the electrode 3.
Meanwhile, FIG. 4 is a cross-sectional view taken along line “A–A′” shown in FIG. 3. A substrate 5 shown in FIG. 4 is made of p-Si or the like. A core layer 6 is a p-SiGe layer and formed on the substrate 5. Further, most of incident light is waveguided and propagated in this core layer 6. Further, the “X-shaped” optical waveguide 2 is formed in the core layer 6. An n+-region 7 for a contact is formed in the intersecting portion of the optical waveguide 2. A p+-region 8 for a contact is formed in the vicinity of the intersecting portion.
An insulating film 11 is made of SiO2 or the like and formed on a part of the core layer 6, which is other than the n+-region 7 and the p+-region 8. An n-electrode 9 is formed on the n+-region 7. A p-electrode 10 is formed on the p+-region B.
Next, an operation of the example of the conventional optical switch shown in FIGS. 3 and 4 is described hereinbelow.
In a case where the optical switch is in “OFF”-state, no electric current is supplied to the electrode 3 (or the electrode 9) and the electrode 4 (or the electrode 10). Thus, change in the refractive index of the intersecting portion of the “X-shaped” optical waveguide 2 shown in FIG. 3 does not occur. Consequently, for example, an optical signal having been incident from an incident end designated by “PI01” in FIG. 3 goes straight through the intersecting portion and is outputted from an output end designated by “PO01” in FIG. 3.
Conversely, in a case where the optical switch is in “ON” state, electric current flows from the p-electrode 10 to the n-electrode 9 through the n+-region 7. That is, electrons are injected from the electrode 3 (or the electrode 9), while hole are injected from the electrode 4 (or the electrode 10). Thus, carriers (electrons and holes) are injected into the intersecting portion. Consequently, the carrier density of a part of the optical waveguide, which is located near to the n+-region 7, is increased.
This increase in the carrier density results in reduction in the refractive index of the intersecting portion of the “X-shaped” optical waveguide 2 shown in FIG. 3. For instance, an optical signal having been incident from the incident end designated by “PI01” shown in FIG. 3 is totally reflected at the boundary between a low-refractive-index region (that is, the region located near to the n+-region 7 in the optical waveguide 2), which is produced in the intersecting portion, and a region (the remaining half of the region in the optical waveguide 2), of which the refractive index hardly changes, and then outputted from an output end designated by “PO02” in FIG. 3.
Consequently, the refractive index of the intersecting portion is controlled by supplying electric current to the electrode to thereby inject carriers (electrons and holes) into the intersecting portion of the optical waveguide 2. Thus, a position, from which an optical signal is outputted, can be controlled. In other words, the transmission path, through which an optical signal is propagated, can be switched.
Therefore, a light reflection region can efficiently be produced by clearly defining the boundary between the region, in which the carrier density is increased and the refractive index change is caused by injecting an electric current thereinto, and the region, in which the refractive index change does not occur, in the optical waveguide 2 to thereby enable occurrences of light reflection thereat.
Additionally, a principle on the basis of the change of the refractive index that results from the shift of the wavelength of a light absorbing end in the inter-band transition of a semiconductor material (a band filling effect) (See, for example, Document (2) referred to below) or the refractive index change due to the carrier density is caused on the basis of a plasma dispersion effect (See, for example, Document (3) referred to below.). Thus, in a case where the carrier densities of two optical waveguides are equal to each other, the change in the refractive index of one of the two optical waveguides, which is smaller in the effective mass of carriers (that is, free electrons and free holes) than the other optical waveguide, is larger than that in the refractive index of the other optical waveguide. Thus, large change in the refractive index is caused at a smaller amount of injected electric current (that is, at a lower current density) by using a material system, which is small in the effective mass of carriers. Consequently, a low-current-driven optical switch can be realized.
Referring next to FIG. 5, there is shown an example (See, for example, Document (4) referred to below.) of a cross-sectional view of a conventional optical switch using a material system, which is small in the effective mass of carriers (free electrons and free holes).
A substrate 12 shown in FIG. 5 is made of InP or the like. A core layer 13 is constituted by, for instance, an n-InGaAsP four-element layer and formed on the substrate 12. An n-InP layer 14 is formed on the core layer 13. An n-InGaAsP layer 15 is formed on the n-InP layer 14. An insulating film 16 is made of SiO2 or the like and formed on the n-InGaAsP layer 15. A p-electrode 17 is formed on the insulating film 16. An n-electrode 18 is formed on the back surface of the substrate 12.
The optical switch shown in FIG. 5 is configured so that the core layer 13, the InP layer 14, and the InGaAsP layer 15 are serially formed in this order on the substrate 12, and that an “X-shaped” optical waveguide is formed by etching down to the core layer 13.
Further, p-type impurities are diffused in a portion designated by “DR11” in FIG. 5. Then, an electrode 17 is formed in such a way as to be in contact with the portion designated by “DR11” in FIG. 5. An electrode 18 is formed on the back surface of the substrate 12.
According to the example of the conventional optical switch shown in FIG. 5, larger change in the refractive index is obtained at a lower current injection amount (or at a lower current density) by using a material system, which is small in the effective mass of carriers (free electrons and free holes). Consequently, a low-current-driven optical switch can be realized.
Next, a description is given of an optical switch constituted in such a way as to limit a refractive index change region by current confinement. FIGS. 6 and 7 are a plan view and a cross-sectional view illustrating an example of the conventional optical switch, in which a p-type region is provided in the intersecting portion of the optical waveguides thereby to perform current confinement, to confine a high-carrier-density region, and to limit the refractive index change region (See, for example, Document (5) referred to below.).
As shown in FIG. 6, an “X-shaped” optical waveguide 20 is formed in a semiconductor substrate 19. An electrode 21 is formed at the intersecting portion of the “X-shaped” optical waveguide 20.
Meanwhile, FIG. 7 is a cross-sectional view taken along line “B–B′” shown in FIG. 6. A substrate 22 shown in FIG. 7 is made of, for instance, InP. A lower clad layer 23 is made of, for example, an n-InP layer and formed on the substrate 22. A core layer 24 is an n-InGaAsP layer and formed on the lower clad layer 23. A contact layer 26 is an n-InGaAsP layer and formed on the upper clad layer 25.
In portions designated by “DR31” to “DR33” in FIG. 7, Zn, which is p-type impurity, is diffused. Oxide film 27 is made of SiO2 or the like and formed on a part of the contact layer 26, which is other than the diffusion region designated by “DR33” in FIG. 7. A p-electrode 28 is formed on the diffusion region designated by “DR33” in FIG. 7. An n-electrode 29 is formed on the back surface of the substrate 22.
Hereunder, an operation of the conventional optical switch shown in FIGS. 6 and 7 is described. In a case where the optical switch is in “OFF”-state, no current is supplied to the electrode 21 (or the electrode 28) and to the electrode (not shown), which is formed on the back surface 19 (and corresponds to the electrode 29 shown in FIG. 7).
Thus, no change in the refractive index of the intersecting portion of the “X-shaped” optical waveguide 20 occurs. Therefore, for example, an optical signal having been incident from a portion designated by “PI21” in FIG. 6 goes straight in the intersecting portion and outputted from a portion designated by “PO21” in FIG. 6.
Meanwhile, in a case where the optical switch is in “ON”-state, currents are supplied to the electrode 21 (or the electrode 28) and an electrode (not shown), which is provided on the back surface of the substrate 19 (and corresponds to the electrode 29 shown in FIG. 7). Further, carriers (electrons and holes) are injected into the intersecting portion.
Thus, the refractive index of a portion located just under the electrode 21 provided at the intersecting portion of the “X-shaped” optical waveguide 20 is changed through the influence of a plasma effect in such a way as to become lower. Therefore, an optical signal having been incident from an end designated by “PI21” in FIG. 6 is totally reflected by the boundary between a low refractive portion, which is produced in the intersecting portion, and the remaining portion thereof and outputted from a portion designated by “PO22” in FIG. 6.
Consequently, the position from which an optical signal is outputted, in other words, a transmission path, through which the optical signal is propagated, can be switched by supplying electric current to the electrodes; so that carriers (electrons and holes) are injected into the intersecting portion of the “X-shaped” optical waveguide 20, thereby to control the refractive index of the intersecting portion.
The following documents (1) to (5) are referred to as related art.
(1) Baujun Li, Guozheng Liu, Zuimin Jiang, Chengwen Pei, and Xun Wang: “1.55 μm Reflection-Type Optical waveguide Switch Based on SiGe/Si Plasma Dispersion Effect”, Appl. Phys. Lett., Vol. 75, No. 1, pp. 1–3, 1999.
(2) Applied Physics Society, Editor of Optical Social Meeting, “Optical Integrated Circuit -Basis and Application-”, First Edition, Asakura Bookshop, Apr. 10, 1988, Chapter 5, p 104
(3) Baujun Li, and Soo-Jin Chua: “2×2 Optical Waveguide Switch with Bow-Tie Electrode Based on Carrier-Injection Total Internal Reflection in SiGe Alloy”, IEEE Photon. Tech. Lett., Vol. 13, No. 3, pp. 206–208, 13 (2001).
(4) Hiroaki Inoue, Hitoshi Nakamura, Kenichi Morosawa, Yoshimitsu Sasaki, Toshio Katsuyama, and Naoki Chinone: “An 8 mm Length Nonblocking 4×4 Optical Switch Array”, IEEE Journal on Selected Areas in Communications, Vol. 6, No. 7, pp. 1262–1266, 1988.
(5) K. Ishida, H. Nakamura, H. Matsumura, T. Kadoi, and H. Inoue: “InGaAsP/InP Optical Switches Using Carrier Induced Refractive Index Change”, Appl. Phys. Lett., Vol. 50, No. 19, pp. 141–1442, 1987.
According to the example of the conventional optical switch described above, light reflection is caused by the boundary between a region, in which a carrier density is increased by current injection (or carrier injection) to thereby cause change in the refractive index thereof (or reduce the refractive index thereof), and a region, in which no change in the refractive index thereof occurs, thereby to perform optical switching and to switch the transmission path of an optical signal.
Therefore, the conventional optical switch operates according to a principle based on the refractive index change due to a plasma dispersion effect (See Document (3)) or according to a principle based on a refractive index change (due to a band filling effect), which arises from shift of the optical absorption edge wavelength at an interband transition of a semiconductor material (see Document (2)).
However, the optical response speed of the optical switch for optically switching the transmission path of an optical signal according to a charier density change is restricted by a carrier life. Therefore, the optical response speed is also determined by the carrier life that depends upon the semiconductor material and the structure of a current injection region, that is, a refractive index change region of the optical switch. For example, the optical response speed of the conventional optical switch is several tens nanoseconds to several hundreds nanoseconds due to the carrier life when the injection of a drive current is stopped. Thus, the optical response speed of the conventional optical switch is low.